3.6 supports only el7 hosts.
Cross-cluster-migration from el6 hosts to el7 hosts is supported only on 3.5.
It's not possible (easily, at least) to upgrade hosted-engine hosts from el6 to el7 after starting upgrading to 3.6.
engine-setup of 3.6 now checks if it's running as a hosted-engine on el6 hosts, and if so, suggests to first upgrade them to el7.
The upgrade process can be finished successfully.
Note that this is also tracked in bug 1188679. Feel free to drop current from RN and/or write a very short text only mentioning engine-setup (mainly so that people that stumble upon this when running it will be less surprized).